The present invention relates to signal processors for correcting signal distortion by shaping the frequency spectrum and/or the amplitude-density function of the signal.
Signals transmitted from one point to another are subject to a number of distortion mechanisms.
One classical distortion mechanism is linear, or convolutional, distortion. A linear method of correction, called predictive deconvolution, has been developed to remove convolutional distortion from the signal by whitening the frequency spectrum of the signal. This method is described in U.S. Pat. No. 4,000,369, entitled "Analog Signal Channel Equalization With Signal-in-Noise Embodiment" (James E. Paul Jr. et al., Dec. 28, 1976) and U.S. Pat. No. 4,052,559, entitled "Noise Filtering Device" (James E. Paul Jr. et al., Oct. 4, 1977). In the method described in the second of those patents, additional spectrum shaping is done by a fixed passive filter in tandem with the predictive deconvolver to restore a natural shape to the spectrum. U.S. Pat. No. 4,507,741, "Adaptive Spectrum Shaping Filter" (Stanley A. White, Mar. 26, 1985) describes a more powerful filter for actively shaping the frequency spectrum of an input signal from any given initial form to any desired output form. Such processing is also linear.
A second distortion mechanism that can severely corrupt a transmitted signal is nonlinear distortion. U.S Pat. No. 4,315,319, "Nonlinear Signal Processor" (Stanley A. White and V. A. Vitols Feb. 9, 1982) describes a nonlinear signal-processing technique to combat the problem of nonlinear distortion by mapping an input signal having any given amplitude-density function into a new signal with any desired amplitude-density function, subject only to the constraint that the distorting mechanism be a single-valued nonlinear function, the slope of which is everywhere positive. The nonlinear signal-processing technique of that patent has been successfully applied to both speech and image-restoration problems.
Nevertheless, some problems in actual signal transmission remain. Most signals are in fact subjected to both convolutional (linear) and nonlinear distortion. Yet, it has been found that attempting to restore the signal by coupling two correction devices together in series has not worked successfully. A linear filter followed by a nonlinear filter shapes the frequency spectrum first, then corrects the amplitude-density function. Unfortunately, shaping the amplitude-density function changes the frequency spectrum established by the linear filter. A nonlinear filter followed by a linear filter corrects the amplitude-density function first, then shapes the frequency spectrum. However, the frequency-shaping function of the linear filter modifies the desired amplitude-density function established by the nonlinear filter, except in the case of a Gaussian-shaped density function.